Conditions
Acanthosis nigricans: dark skin on the neck and armpits
Acanthosis nigricans is darker, thicker, velvety skin in body folds, mostly on the neck, armpits and groin. It is not dirt and does not scrub off: it is often a sign of insulin resistance, which can come before diabetes. The doctor evaluates you by video and gives you a lab order to check your blood sugar and cholesterol at a lab near you.
Is it an emergency? Call 911.
Is this an emergency?
This is not an emergency service. Call 911 or go to the nearest emergency room if you have:
- Extreme thirst and urinating a lot, with vomiting or great tiredness
- Confusion, heavy sleepiness or fast, deep breathing
- Fruity-smelling breath, with belly pain or vomiting
- Dark skin that appears suddenly and spreads fast in an adult (prompt in-person evaluation)
Help lines
At a glance
Acanthosis nigricans
Common symptoms
- Darker skin on the neck, armpits or groin
- Thick, velvety skin to the touch
- Sometimes, dark skin over the knuckles
- Other names
- Dark neck
- Usual test
- Hemoglobin A1c, fasting glucose and cholesterol
- Treatment
- Treating the cause, such as insulin resistance or diabetes
On this page(8)
What is acanthosis nigricans?
Acanthosis nigricans is a skin change: the skin becomes darker, thicker and velvety in body folds. According to MedlinePlus, it appears in the armpits, groin and neck folds and over the joints of the fingers and toes; sometimes on the lips, palms or soles.
It is more common in people with insulin resistance, diabetes or obesity, and in some genetic conditions. Hormone changes and some medicines, such as growth hormone or birth control pills, can also cause it. Sometimes it appears in otherwise healthy people.
In Puerto Rico, 14.4% of adults had diagnosed diabetes in 2021, the highest of any US state or territory, according to the CDC. That is why a "dark neck" in a child or an adult is a good reason to check blood sugar. See prediabetes.
How we help by video
You see Dr. Diana Nieves Castro, a Puerto Rico–licensed general practice physician with several years of emergency-department experience, in English or Spanish. The doctor looks at the skin on camera (you can send photos through the secure link if she asks) and asks how long you have had it, what medicines you take and whether diabetes runs in your family.
We evaluate, diagnose and treat. What matters is finding the cause: the doctor gives you a lab order for a lab near you, for example hemoglobin A1c, fasting glucose and cholesterol, and reviews the results with you. If they are off, we help with the plan; see type 2 diabetes and high cholesterol.
We see children of all ages: a parent or legal guardian is on camera for the whole visit and shows photo ID.
Your doctor
Dr. Diana Nieves Castro, MD
General practice physician
Several years of emergency-department experience.
Founding Executive Director of The Health Initiative.
Your visit in 3 steps
Request a visit online 24/7 — we confirm your time by email or text This is how you see a doctor online with us.
Request your visit.
Book a visit and choose "Doctor visit". You pay by card when you request it.
See the doctor by video.
We send you a secure video link. It works on your phone, tablet or computer.
Get your plan.
You get a lab order and, once results are in, the doctor reviews them with you.
What the visit costs
The evaluation is a US$79 doctor visit, the same price for every age, with no membership. Reviewing the results of labs our doctor ordered is a US$49 follow-up visit. The lab charges separately. We do not bill insurance. See prices.
Acanthosis nigricans symptoms
It rarely hurts. What you notice is the skin change:
- Darker skin on the neck, armpits or groin
- Thick, velvety skin to the touch
- Sometimes, dark skin over the knuckles
Treatment: treat the cause, not just the patch
MedlinePlus explains that treating the underlying condition, such as diabetes, is essential. As insulin resistance improves, the skin may lighten over time.
Lightening creams do not treat the cause. Do not use home bleaches or harsh products that irritate the skin. If a medicine might be causing it, talk with the doctor before you stop taking it.
When to get in-person care or go to the ER
If dark skin appears suddenly and spreads quickly in an adult, MedlinePlus says other causes, including cancer, should be considered. That needs a prompt in-person evaluation; the doctor tells you where to go.
For your safety, go to the ER or call 911 if you have signs of very high blood sugar:
- Extreme thirst and urinating a lot, with vomiting or great tiredness
- Confusion, heavy sleepiness or fast, deep breathing
- Fruity-smelling breath, with belly pain or vomiting
How we care for you
We evaluate, diagnose and treat; if needed, we give you a lab order for a lab near you.
We see adults and children of all ages. Patients under 21: a parent with patria potestad (parental authority) or a legal guardian is on camera for the whole visit and shows photo ID.
Real-time video visit. We send you a secure video link. It works on your phone, tablet or computer.
If appropriate, the doctor prescribes and we send your prescription to your pharmacy. We do not prescribe controlled substances.
You must be physically in Puerto Rico during the visit (visitors, tourists and cruise passengers are welcome).
In an emergency, call 911.
Frequently asked questions
Why does my child have a dark neck?
Does acanthosis nigricans go away?
Which tests do I need?
Is acanthosis nigricans contagious?
About this page
Author: The Health Initiative
Medical care provided by Dr. Diana Nieves Castro, MD (PR license 24027)
Disclosure: Founding Executive Director of The Health Initiative
Published: 2026-10-02 · Updated: 2026-10-02
Sources (3)
- Acanthosis nigricans (MedlinePlus, NIH) (opens in a new tab) (accessed 2026-10-02)
- Diagnosed diabetes by state and territory, 2021 (CDC, Preventing Chronic Disease) (opens in a new tab) (accessed 2026-10-02)
- Diabetic Ketoacidosis (CDC) (opens in a new tab) (accessed 2026-10-02)